一种考虑平均应力松弛的汽轮机叶根低周疲劳寿命预测方法

LCF Life Prediction Method of Turbine Blade Roots Considering Mean Stress Relaxation Effect

针对汽轮机叶片叶根在实际服役中承受脉动离心载荷以及叶根缺口存在平均应力松弛等特点,开展了叶根材料在应变比R=0下的低周疲劳试验,获得了该材料的平均应力循环松弛规律,并基于Landgraf模型进一步确定了叶根材料的平均应力松弛描述模型。应用局部应变法原理,采用统一模型计算叶根危险部位的局部应力/应变,并采用考虑平均应力松弛的SWT修正模型计算疲劳损伤,建立起一种预测汽轮机叶片叶根低周疲劳寿命的新方法。最后,通过叶根模拟件的低周疲劳试验验证了本文建立的寿命预测方法的有效性。研究结果表明,在叶根低周疲劳设计时,考虑平均应力松弛能够显著提高叶根的疲劳寿命预测精度。

Based on the facts that the turbine blades are subjected to centrifugal loadings with pulsation feature in actual service conditions and there exists the mean stress relaxation at blade root, LCF(low cycle fatigue) tests of the blade root material were carried out under strain ratio R=0 firstly, and then the rule of the cycle-dependent mean stress relaxation of the material was obtained and further described using Landgraf model. Based on the principle of the local strain method, a new approach to predicting the low cycle fatigue life of turbine blade roots was established by using the unified model to calculate the local stress/strain in the dangerous locations of the blade root, and taking into account the mean stress relaxation in the SWT model to calculate the fatigue damage. The effectiveness of the proposed approach was then verified by low-cycle fatigue tests of the blade root specimens. The results show that considering the mean stress relaxation can improve significantly the accuracy of the fatigue life prediction of turbine blade root.